A zone plate is one of the conventional optical elements for diffraction, and is often used to focus an X-ray and act as an objective lens of an imaging system. In fact, a zone plate is a circular optical element for diffraction, which consists of concentric zones, which have radially increasing linear densities and represent alternate bright and dark as shown in FIG. 1). According to the Fresnel's diffraction theory, if a circular aperture is divided to zones in series in accordance with the formula rn=√{square root over (nλf)} (n: serial number of a zone, λ: wavelength; f: focal length), and the odd zones or even zones are shielded, a Fresnel zone plate will thus configured. The zones forming the zone plate is referred to as half-wave zones, wherein the difference between the optical paths from the focal point to arbitrary two adjacent half-wave zones is λ/2. This means that the light waves through two adjacent half-wave zones have a phase difference of π when they arrive the focal point, and destructive interference will occur at the focal point. Hence, a zone plate consists of alternate transparent and opaque half-wave zones, so as to achieve the purpose of constructive interference. If opaque half-wave zones in a zone plate are replaced by transparent half-wave zones having a phase shift of II, a transparent half-wave zone having an optical path of λ/2 will be made, and thus the light waves through two adjacent half-wave zones have a phase difference of either 0 or 2π, when they arrive the focal point, and all of the light passing through the zone plate will form constructive interference at the focal point, which is the theory of normal phase type zone plates. [See Synchrotron Radiation Light Source and Application Thereof, final volume, p. 686]
The two most important parameters of a zone plate are the outermost zone width and the zone thickness. The spatial resolution (Δ) of a zone plate imaging system is determined by the outermost zone width (drN) of the zone plate, by A=1.22drN. In a certain extension, the diffraction efficiency of a zone plate is improved with increasing zone thickness. However, an important factor that limits the development of zone plates is the ratio of the height to the width. There is a conflict between the outermost zone width and the zone thickness in a zone plate due to the processing technology, among other factors. Currently, either the outermost zone width of a zone plate cannot be smaller, or the efficiency of the high resolution zone plate cannot be very high. Therefore, there is a bottleneck in the spatial resolution of zone plate imaging systems (hard X-ray: 30 nm; water window: 10 nm).
The refractive index of a substance may be represented as n=1−δ+1β, wherein δ is a phase term, β is an absorption term. At some certain energies, δ and β of a substance would change suddenly, and further, the phase term δ therein may have a negative value. Presence of a phase shift having a negative value possibly bring new breakthrough for the phase type zone plates, which are made by utilizing phase-shift characteristics of elements.